1. Field of the Invention
The present invention relates to an ink jet recording head and an ink jet recording method which are applied to an ink jet printer, particularly a bubble jet printer using bubbling phenomena, and the like.
2. Related Background Art
A bubble jet recording method is a recording method in which ink in a liquid path is locally heated to a high temperature by heating means to bubble the ink, pushing the ink out of a discharge port by a high pressure generated during bubbling, and allowing the ink to be deposited to a recording medium such as a recording paper and the like. A recording head applied to a bubble jet recording method generally includes fine structures such as a discharge port, a liquid path, heating means provided at a portion of the liquid path and used as an ink discharge energy generation portion, and the like.
In such bubble jet recording methods, as an ink jet recording head which discharges ink from discharge ports provided in a matrix in accordance with heating means, with heating means arranged longitudinally and transversely in a matrix in plural numbers, Japanese Patent Application Laid-Open No. 64-20151 discloses an ink jet recording head which disposes a plurality of longitudinal wiring and transverse wiring on a substrate, and includes a rectifying device through which an electric current is flowed only when a forward voltage is applied and an electric current is not flowed by the application of reverse voltage, and a heating resistor used as heating means connected to the rectifying device on the crossing portions of the longitudinal wiring and the transverse wiring. In this ink jet recording head, a voltage is applied to any of longitudinal wiring and any of transverse wiring to apply a forward voltage to a heating resistor on the crossing portion of the longitudinal wiring and the transverse wiring, so that this heating resistor is driven. In this case, to other heating resistors is flowed no electric current while keeping states where no voltage is applied thereto, or a reverse voltage is applied thereto. Thus, a predetermined heating resistor of a plurality of heating resistors is driven.
Further, Japanese Patent Application Laid-Open No. 57-36679 discloses an ink jet recording head in which a diode, which is heated by the energization of forward current and which a reverse current cannot be energized, is used as heating means, and these diodes are arranged in a matrix in the plural numbers on a substrate.
Further, in an ink jet recording head, an about 0.05 xcexcm thick tantalum nitride (TaxNy) thin-film resistor is usually used as heating means, and ink is heated and bubbled by the Joule heat generated when electric current is energized through this thin-film resistor. On such a thin-film resistor is usually provided an about 0.2 xcexcm thick cavitation-resisting layer composed of a metal such as Ta or the like through an about 0.8 xcexcm thick insulator of SiN or the like, to prevent damage of the surface of the thin-film resistor due to cavitation. As a method of forming the cavitation-resisting layer, a method of laminating three protective layers on a heating resistor, and the like have been proposed (refer to Japanese Patent Application Laid-Open No. 5-301345 and the like).
To make high definition images recordable by an ink jet recording method, a technique of discharging as minute an ink droplet as possible with high density is required. For the sake of this, it is basically important to form a fine liquid path and fine heating means.
As a method of forming such fine structures, a method of producing a high density recording head using the photolithography technology has been proposed, which utilizes the simplicity of the recording head structure in a bubble jet recording method (refer to Japanese Patent Application Laid-Open No. 8-156269 and the like).
Further, as a method of discharging a minute or fine ink droplet, the method using heating means having the heat value larger at the center portion compared to that in the edge portions has been proposed (refer to Japanese Patent Application Laid-Open No. 62-201254).
In a conventional ink jet recording head, when a plurality of heating means are selectively driven, a noise voltage is applied to non-selected heating means to waste energy and affect the driving voltage thereby to change the discharging amount of ink and possibly affect recording images. Particularly, in an ink jet recording head, in which a voltage is applied to a plurality of longitudinal wiring and a plurality of transverse wiring so that heating means provided in a matrix on the crossing of the longitudinal wiring and the transverse wiring are selectively driven, a voltage lower than the driving voltage is possibly applied to non-selected heating means, and when this voltage is applied in the forward direction, non-required heat is generated in non-selected heating means.
When a cavitation-resisting layer is provided on a heating resistor, the heating resistor indirectly contacts ink through the cavitation-resisting layer, and the cavitation-resisting layer prevents the heat transfer to the ink, whereby the heat is difficult to transfer to the ink. Thus, the application of electric energy is further required compared to a case where a cavitation-resisting layer is not provided, and the consumption electric power may be increased.
On the other hand, another method is known, in which a cavitation-resisting layer is not provided and a layer itself composed of a metal having the cavitation-resisting properties is used as a heating resistor. However, since the specific resistance of the metal having the cavitation-resisting properties is usually low (for example, the specific resistance of Ta is 2xc3x9710xe2x88x926 xcexa9cm), when this metal film thickness is increased, the area of a conducting path is increased and the resistance is excessively reduced, whereby the function of the heating resistor is not effected. Therefore, in a method in which a layer itself composed of a metal having the cavitation-resisting properties is used as a heating resistor, to use the metal layer having a sufficiently increased film thickness is difficult.
Further, when a cavitation-resisting layer having a decreased film thickness is formed to improved the heat transfer, the functions of a heating resistor are possibly lost by the generation of a pin-hole due to cavitation or the like.
Further, in a case where the size of a heater is decreased to allow a fine ink droplet to discharge, when the heater portion and the wiring portion connected to the heater are miniaturized in the same ratio as the heater is miniaturized, an increase of the wiring resistance is invited. Thus, the miniaturization of the heater portion and the like has limitations.
Further, although the photolithography technology is excellent in the working precision, it is inferior in a mass production. On the contrary, although the printing technology is advantageous in a mass production, it is inferior in the working precision. That is, when a heater or the like is finely formed for the purpose of discharging a minute liquid droplet, it is difficult to satisfy both of the precision and mass productivity.
One object of the present invention is to provide an ink jet recording head providing heating means which does not generate non-required heat even though a voltage such as a noise voltage lower than the driving voltage is applied to the heating means.
Further, another object of the present invention is to provide an ink jet recording head which has a resistance to cavitation and has a small consumption electric power.
Still another object of the present invention is to provide a liquid discharge device which can heat a minute spot-shaped area even when a heater film area cannot be formed in a sufficiently small size, and can discharge a minute liquid droplet, and is to provide an ink jet recording head and an ink jet apparatus, which have this liquid discharge device and can perform high precision recording.
An ink jet recording head according to the present invention, which discharges an ink droplet using the thermal energy from heating means is characterized in that the heating means is a heating device having MIM (Metal-Insulator-Metal)-type current-voltage properties in which the resistance value when a low voltage is applied shows a higher value compared to the resistance value when a high voltage is applied, regardless of the polarity.
By using a heating device having such an MIM-type current-voltage properties, even though a voltage such as a noise voltage lower than the driving voltage of the heating device is applied to the heating device, the resistance value in the heating device when a low voltage is applied is large. Thus, the electrical current hardly flows in the heating device. Therefore, non-required heat in the heating device can be prevented.
It is preferable that the current-voltage properties of a heating device are those in which current flowed by the application of voltage of a small absolute value is sufficiently small on both the positive voltage side and the negative voltage side so that the heating device is not heated by the noise voltage having an unstable polarity. Thus, it is desirable that in the current-voltage properties of a heating device, particularly, the ratio (V1/V2) of absolute values of an applied voltage +V1 to an applied voltage xe2x88x92V2 which provide current of an absolute value I0 corresponding to the current which is flowed through the heating device when the thermal energy required for bubbling ink is generated, is a value of 0.5 or more and 2 or less and that the absolute value of current which is flowed when voltage of +V1/2 or xe2x88x92V2/2 is applied, is I0/10 or less.
Further, an ink jet recording head can be made, which can selectively heat a heating device by selectively applying a voltage to a stripe-shaped electrode by making a structure in which heating devices are arranged in a matrix on the crossing of stripe-shaped electrodes provided longitudinally and transversely so as to cross each other. In this case, even though a voltage lower than the driving voltage of the heating device is applied to some heating devices on non-selected positions, the heating device has MIM-type current-voltage properties. Accordingly, current is hardly flowed through the heating device and the heating device is not heated.
Such a heating device having the MIM-type current/voltage properties includes an MIM device having an insulator and a pair of conductor electrodes sandwiching the insulator therebetween.
This MIM device can have a configuration that an insulator portion is a heating portion, any one of conductor electrodes sandwiching the insulator acts as an ink contact surface and the ink is heated on the surface. Since the heat value of the MIM device depends on only the thickness of the insulator and the material constant, the thickness of the conductor electrode can be increased without loss of the heating properties of the MIM device. Therefore, the MIM device can be used as cavitation-resisting heating means by increasing the thickness of a conductor electrode surface from which ink is bubbled.
To flow current through an MIM device by the tunnel conduction that is the electric conduction mechanism in an insulator of the MIM device, the space between the electrodes is desirably 100 nm or less. Further, it is desirable to set the space between the electrodes to 20 nm or less so that the MIM device is rapidly heated by the application of a pulse voltage of a short pulse width such as about 1 xcexcsec.
Further, when the space between the electrodes is extremely narrow, ions on the metal surface of the electrode possibly causes the field radiation. Thus, the space between the electrodes is desirably 1 nm or more. Additionally, setting the space between the electrodes to 4 nm or more is more desirable so that a stable tunnel conduction can be obtained.
Further, by forming a conductor electrode surface, from which ink is bubbled, with a metal having cavitation-resisting properties, the cavitation-resisting properties of heating means can be enhanced.
In a conventional ink jet recording head in which a metal having the cavitation-resisting properties is formed on heating means, heat of the heating means is transmitted to ink through the metal having the cavitation-resisting properties and an insulating layer sandwiched between this metal layer and the heating means to electrically insulate them. On the other hand, in an ink jet recording head of the present invention, heat of a heating portion is transmitted only through a metal having the cavitation-resisting properties. Accordingly, the thermal conductivity is improved, and ink can be efficiently bubbled with a reduced consumption electric power.
Further, by forming an oxide insulating film on a conductor electrode surface the durability of the conductor electrode is enhanced and the electrode is further resistant to heat.
As a structure of an MIM device, the MIM device can employ a structure which has a pair of thin-film conductive electrodes spaced on a substrate surface which, at least, is composed of an insulator. That is, in this structure, the space between the conductor electrodes acts as an insulator which causes the tunnel conduction. An insulator may be provided in this space.
As another structure of the MIM device, the MIM device can employ a laminate structure which has a conductor electrode of a metal thin-film disposed on a substrate, a thin-film insulator disposed on the conductor electrode and a conductor electrode of a metal thin-film disposed on the insulator.
An ink jet recording head using an MIM device having the laminate structure can employ a configuration in which heating means are easily disposed in a matrix by forming a configuration having stripe-shaped lower electrodes formed in parallel in a plurality of lines, striped-shaped upper electrodes formed in parallel in a plurality of lines so as to cross the stripe-shaped lower electrodes, and insulators each sandwiched between the stripe-shaped lower electrode and the stripe-shaped upper electrode, and ink is discharged from matrix-shaped discharge ports. That is, in this configuration, an MIM device is formed at the crossing or intersection formed by a stripe-shaped lower electrode and a stripe-shaped upper electrode.
The drive of the MIM device arranged in the matrix is performed by applying a voltage to a stripe-shaped lower electrode and a stripe-shaped upper electrode which pass through an MIM device portion to be driven, to apply an electric potential difference of the voltage applied to both electrodes, to the MIM device. In this case, although an electric potential difference is imparted to the MIM device in which a voltage is applied to only any one of the stripe-shaped upper electrode and the stripe-shaped lower electrode, this electric potential difference is smaller than the electric potential difference applied to the MIM device in which a voltage was applied to both the electrodes. In the MIM device, the amount of change in current is larger than the amount of change in electric potential difference applied. Therefore, even though an electric potential difference smaller than the electric potential difference by which a desired heat value is obtained is applied, current is hardly flowed and the MIM device is hardly heated. That is, a non-required heat value of non-selected MIM device is small.
The ink jet recording head according to the present invention has no necessity to separately provide voltage applying means on each of the MIM devices arranged in a matrix as mentioned above. Thus, a configuration that the voltage applying means for the MIM device is provided in the external portion of the ink jet recording head and that voltage applying means and detachable interface electrode portions are formed in the ink jet recording head, can be easily obtained. That is, the end portions of the stripe-shaped lower electrode and stripe-shaped upper electrode are disposed at the outer regions of the ink jet recording head, so that this outer regions may be used as the external voltage applying means and the detachable interface electrode portion. By forming such a configuration, the manufacturing costs of the ink jet recording head which is necessary to replace due to the consumption of the ink can be reduced.
Other heating devices having the MIM-type current-voltage properties include a varistor in which a sintered body layer to which a metal oxide was added or granular crystal layer was sandwiched with a pair of conductors.
Further, other heating devices having the MIM-type current-voltage properties include a surface conduction type device in which a thin-film resistor, particularly, a fine particle thin-film resistor composed of dispersedly-disposed fine particles, or an island-shaped thin-film resistor was disposed between a pair of device electrodes. This surface conduction type device can employ a configuration that a thin-film resistor having a linear field-concentrated region during the energization of current is used. By using such a surface conduction device as heating means, the thin-film resistor can form the linear field-concentrated region during the energization of current. Accordingly, even though this thin-film resistor is composed of a film having a comparatively large area, the minute spot heating in the field-concentrated region can be effected, whereby a fine liquid droplet can be discharged.
An ink jet recording device according to the present invention is characterized by comprising at least the above-mentioned ink jet recording head provided with ink discharge ports for discharging ink facing the surface of a recording medium, and recording media carrying means.
According to the ink jet recording head of the present invention, non-required heat due to a noise voltage value lower than the driving voltage and the like can be prevented by using a device having MIM-type current-voltage properties as heating means.
Further, according to the present invention, by using the MIM device as heating means in an ink jet recording head, the heating portion can be protected from cavitation, the distance between the heating portion and the ink contact surface can be decreased. Therefore, an ink jet recording head, which is resistant to the cavitation and has a reduced consumption electric power, can be formed.
Further, according to the present invention, by using a surface conduction device characterized by comprising a thin-film resistor disposed between a pair of element electrodes, the thin-film resistor having a linear field-concentrated region during the energization of current, as heating means, even when a film area cannot be formed in sufficiently small size, minute spot heating in the field-concentrated region can be effected, whereby a fine liquid droplet can be discharged.